Matte/Gloss would be nice and easy (as they're already sitting ready to go)
Although it'd be rather difficult, someone suggested to me patterned dyes (like the militia harness/subligar) using a big-cat print.
Lots of room for exansion (however some might be more difficult than others)
However material layers are much easier to implement in games now, applying a "tiled pattern" effect to gear might be rather easy (just a time consuming task to get right). Could make for some rare/interesting dyes on the market (similar to Guild Wars 2 where true Black/White dyes are a fortune).